1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and particularly to a semiconductor integrated circuit device having an open-drain-type output circuit in which an N-channel transistor has its source connected to a reference potential point and has its drain connected to an output terminal.
2. Description of the Prior Art
Conventionally, a semiconductor integrated circuit device is typically provided with an output circuit of a CMOS type as shown in FIG. 3, or of an open-drain type as shown in FIG. 4. In FIG. 3, the output circuit has an input terminal 90 that is connected to a driving circuit provided within the semiconductor integrated circuit device.
This circuit has a P-channel MOS transistor 102 of which the gate is connected to the input terminal 90, the source is connected to a power source line 101, and the drain is connected to an output terminal 100. The circuit also has an N-channel MOS transistor 103 of which the gate is connected to the input terminal 90, the source is connected to ground, and the drain is connected to the output terminal 100. In this circuit, when a high level is fed to the input terminal 90, the transistor 102 is turned off, and the transistor 103 is turned on, causing a low level to appear at the output terminal. By contrast, when a low level is fed to the input terminal 90, the transistor 102 is turned on, and the transistor 103 is turned off, causing a high level to appear at the output terminal 100.
On the other hand, in FIG. 4, the output circuit has an input terminal 90, and has an N-channel MOS transistor 104 of which the gate is connected to the input terminal 90, the source is connected to ground, and the drain is connected to an output terminal 100. In this circuit, when a high level is fed to the input terminal 90, the transistor 104 is turned on, causing a low level to appear at the output terminal 100. However, when a low level is fed to the input terminal 90, the transistor 104 is turned off, and this brings the output terminal 100 into an electrically floating state.
When a person touches the output terminal 100 with his or her hand while the circuit is not in operation, or for some other reason, the output terminal 100 may receive an abnormally high electrostatic voltage. In the circuit shown in FIG. 3, where the power source line 101 is kept at the ground level when the circuit is not in operation (i.e. not in use), such an electrostatic voltage causes punch-through, which allows a current to flow through the P-channel MOS transistor 102 and then via the power source line 101 to ground. As a result, the incidence of destruction of the transistor is relatively low. This current due to punch-through does not always serve to discharge a sufficient amount of electric charge, and therefore it does not provide sufficient protection for the transistor. Nevertheless, this still helps protect the transistor against an electrostatic voltage unless the voltage is extremely high.
By contrast, in the circuit shown in FIG. 4, where there is no route to bypass (discharge) static electricity, a current destroys the transistor 104 as it flows therethrough to ground. Thus, an output circuit of the open-drain type is said to have low resistance to static electricity.